Design Of Broadband High-order Orbital Angular Momentum Mode Converters Based On Helical Fiber Gratings

The optical fiber mode division multiplexing(MDM)system uses different optical fiber modes to transmit information,which can greatly increase the capacity of the optical communication system.The optical fiber mode converter studied in this paper is an important device to realize the mode division multiplexing system.This paper proposes a design and production method of a high-order orbital angular momentum mode converter based on spiral fiber grating.The research content in the paper is divided into the following parts:In this paper,starting from Maxwell's equations,the dispersion equations and field distribution expressions of fiber modes are deduced,and the mode coupling characteristics of long-period fiber gratings and spiral fiber gratings are analyzed.The effect of the double resonance characteristics near the dispersion transition point on the design of broadband optical fiber devices is studied.The influence of modal dispersion characteristics in the fiber on the design of broadband optical fiber devices is also discussed.Finally,the effects of phase sampling grating and equivalent DC sampling grating theory on the design of special transmission spectrum are analyzed and verified.These analyses and discussions have laid a theoretical foundation for the design of subsequent broadband high-order mode converters.Then the effective refractive index of the core mode in the four-mode fiber is calculated,combined with the dual resonance characteristics near the dispersion conversion point and the principle of phase sampling,based on the helical fiber grating,a broadband flat-top mode converter that can realize the mode coupling of LP₀₁ and LP₂₁ is designed.In this paper,the method of changing the dispersion conversion point by thinning the fiber is further used to realize the design of the mode converter with the wavelength range of 1529 nm to 1644 nm,the conversion efficiency of the designed converter is greater than 99%(-20 d B),and the in-band fluctuation is less than 3 d B.In this paper,the effects of fiber grating manufacturing parameters such as the length of the grating period,the number of periods and the depth of refractive index modulation on the performance of the designed device are analyzed.After that,a broadband flat-top mode converter that can realize the coupling of LP₁₁ and LP₂₁ modes is designed based on the helical fiber grating using the sameMethod,the working band of the designed mode converter is 1475 nm to 1610 nm wavelength range,the conversion efficiency is greater than 99%(-20 d B),and the in-band fluctuation is less than 3 d B.Finally,the production of helical fiber grating and the detection method of coupling mode are introduced in detail,and it is proved through examples that the helical long period grating based on single-mode fiber can output the first-order orbital angular momentum mode,helical long-period grating based on four-mode fiber can output second-order orbital angular momentum mode.Finally,a method to control the output power of the heating carbon dioxide laser and adjust the quality of the straightened fiber weight is proposed,and the fabricated spiral fiber grating is thinned to 108?m.